In automated clinical apparatus used in the analysis of blood, sera and body fluids, such as those manufactured by Abbott Laboratories and Ortho Clinical Diagnostics, Inc., among others, it is common place to aspirate a quantity of fluid sample or reagent into a disposable, usually plastic metering tip. The tip transports fluid to another location within the analyzer and then subsequently dispenses the fluid into a reaction vessel such as a cuvette, for subsequent incubation and analysis. Such metering tips are typically cylindrically shaped with a narrowed nozzle at the bottom end thereof, the design being essentially unchanged from tips commonly used on hand-held pipettes. Advancing analyzer technology has moved toward higher speed and more sensitive monitoring of metering events. Other advances require functions beyond simple aspirating and dispensing of fluids. To that end, traditional tip designs are no longer adequate.
After aspiration of fluid into the metering tip and for the remaining steps of a typical metering cycle, fluid is supported in the tip by a
After aspiration of fluid into the metering tip and for the remaining steps of a typical metering cycle, fluid is supported in the tip by a combination of forces that counteract the weight of the fluid column. These forces include the following:                i) a slight vacuum that is supplied by the metering system;        ii) surface tension effects of the upper fluid meniscus acting on the internal bore of the tip; and        iii) surface tension effects of the lower fluid meniscus acting on the nozzle of the tip.        
The interaction between the fluid meniscus and the nozzle of the tip is a significant portion of the above-noted force balance. The intersection of the bore of the tip with the end of the tip (subsequently referred to as the tip “land”) usually consists of a sharp edge, as does the intersection of the land of the tip with the external nozzle. This geometry is more than merely a convenience to the overall manufacturing process. Surface tension of the fluid interacting with sharp edges of the tip land form resistive energy barriers to fluid meniscus movement. It has been learned that these sharp-edged features of the tip nozzle effectively “latch” the meniscus at either the bore edge or the external nozzle edge of the tip land. Ideally, this “latching” of the fluid meniscus is sufficient to keep the fluid in place in spite of changes in pressure within the metering system and acceleration forces due to transporting the tip within the various stations of the clinical analyzer.
Due to demands for higher throughput of automated analyzers, metering systems need to function at high speeds. This is difficult, in that most metering systems employing disposable tips as metering elements utilize air as the working fluid. Since air is compressible, when the fluid in the tip is accelerated in the vertical direction, forces may be sufficient to break the meniscus “latching” force, causing the fluid column to begin oscillating within the confines of the tip. Once the “latch” has been broken, it is difficult to re-establish, even in the case of analyzer metering systems that have active monitoring and control of internal pressure. Such oscillations can be extremely problematic for a number of reasons. First, and in the case of an analyzer system that dispenses sample onto a dry reagent, having fluid touch the reagent prior to actual fluid dispense can bias the assay result. Second, analyzers having software for detecting the presence of an air bubble in the dispensed volume may experience an increased frequency of errors if the oscillation of fluid in the tip results in the lower fluid meniscus moving up into the barrel of the tip. Third, extreme fluid oscillation can result in loss of fluid from the tip, reducing the volume that is subsequently dispensed into the reaction vessel. In typical automated analyzers, the fluid dispensed is a relatively small volume and must be held to precise tolerances to achieve the needed precision from the analytical result. The loss of even a small amount of sample can bias that assay result.
Given the severity of problems, such as those noted above that are caused by fluid oscillation, it would be useful to have a metering tip that provided features to damper or effectively reduce and/or minimize oscillation.